This invention relates to the cleaning of steam generators, and is more particularly directed to a method of removing sludge deposits from the tube sheet of a steam generator.
A typical steam generator for use in a nuclear electrical generating plant is formed of a vertically-oriented cylindrical shell, a plurality of primary-fluid tubes disposed in the shell so as to form a tube bundle, and a substantially horizontal tube sheet for supporting the tubes so that the tube bundle rises therefrom within the shell.
While the generator can be either a once-through or a U-tube type generator, the method of this invention will be illustrated with the U-tube type generator.
A dividing plate cooperates with the tube sheet to form a primary fluid header at one side of the tube bundle and a primary fluid outlet header at the other side of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator also comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber adjacent the shell, and a feedwater ring is disposed above the pipe-bend end of the tube bundle.
A primary fluid which has been heated by circulation through the core of a nuclear reactor enters the steam generator through the primary fluid inlet nozzle, and is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header and through the primary fluid outlet nozzle to a reactor coolant system. At the same time, feed water is introduced to the steam generator through the feedwater ring. The feed water is conducted down the annular chamber adjacent to shell until the tube sheet near the bottom of the chamber causes the feed water to reverse direction passing in heat-transfer relationship with the outside of the U-types, and up through the inside of the wrapper. While the feed water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the feed water surrounding the tubes, causing a portion of the feedwater to be converted to steam. The steam then rises and is circulated through typical electrical generating equipment.
Because the primary fluid contains radioactive particles, it is of utmost importance to maintain complete isolation between the primary fluid and the feed water. However, the primary fluid is isolated from the feed water only by the walls of the tubes, which are typically constructed from Inconel. It is necessary that the tubes be maintained free of defects so that no breaks will occur in the tube walls. However, under certain conditions, the U-tubes may develop leaks which allow radioactive particles to contaminate the feed water.
There are thought to be at least two causes of such tube leaks in steam generators: one cause being high caustic levels in the vicinity of the tube sheet, and the other cause being tube thinning, also in the vicinity of the tube sheet. Both of the foregoing are associated with sludge that has accumulated on the tube sheet. This sludge is mainly composed of iron oxides and copper compounds, along with traces of other metals that have settled out of the feed water onto the tube sheet. The correlation between sludge levels and tube wall thinning suggests that the sludge deposits provide a situs for concentration of phosphates or other corrosive agents at the tube wall.
Accordingly, it is desirable to remove such sludge deposits periodically and prevent deterioration of the tubes.
One known method for removal of sludge from the tube sheet is the sludge-lance-suction method. This method utilizes high-pressure water to break up and slurry the sludge and also utilizes suction and filtration equipment to remove the water-sludge mixture from the tube sheet. In this known method, a pair of flexible, perforated suction headers are introduced through a six-inch handhole to a position at the periphery of the tube sheet at the peripheral edge of the tube bundle. A multiplicity of small suction openings is provided along the length of each of these suction headers. A jet lance is then introduced through the handhole and is aligned between rows of the tubes. The lance is moved along the tube sheet while two high-velocity water jets are established perpendicular to the movement of the lance. These water jets force the sludge toward the periphery of the tube sheet, where the flexible suction header sucks up the water-sludge mixture.
Experience has shown that this sludge-lance-suction method is not particularly effective. One of the problems with this known method is that an expanded water volume exits the tube bundle near the periphery of the tube sheet and overwelms the capacity of the suction headers. As a result of this, the sludge is redeposited in the wrapper area of the tube sheet or is washed back in amoung the U-tubes. In addition, because the flexible headers need to be provided with an abundance of suction openings, it is difficult or impossible to properly align the holes of the flexible headers with the rows of the tubes on the tube sheet.
In another known method of sludge lancing, high pressure water jets are directed from a fluid lance inserted through a clear passage across the center of the tube sheet toward the periphery through the open passages between rows of the tubes. In this known method, a stream of water is admitted at the periphery of the tube sheet, and is directed so as to flow in a channel circumferentially around the tube sheet in the channel formed between the tubes and the shell, to a removal point also located at the periphery. The high-pressure water jets loosen and entrain the sludge to move the same outward to the peripheral channel. Then, the sludge becomes entrained in the circumferential fluid stream and is theoretically carried away by a suction device at the removal point. In this known method, the jet lance is provided with fluid at as high a pressure as is practicable, using a variable, positive constant-displacement pump, e.g., at 200-10,000 psig, and at as high a flow rate as is practicable. The water to establish the peripheral current is admitted separately through a separate connection at the handhole or through a fixed jet aimed into the space between the tubes and the shell.
Unfortunately, the surge of water emanating from the lance jet adds to the volume in the peripheral current, thereby causing water to pile up ahead of the removal point. This piling up permits sludge particles in suspension to settle out before the suction device at the removal point is reached. Also, the presence of the peripheral fluid current causes the level of fluid to build up on the tube sheet, especially near the periphery thereof. This level of water interferes with the scouring action of the high pressure jet, which is the principal mechanism that removes the sludge from the tube sheet.
In addition, the known methods of removing sludge from the tube sheet rely only upon the impact of a fluid jet and on its continued velocity to loosen sludge particles and to maintain the same in suspension.